Preparation Of Ceria-Based Polishing Powders And Its Application In CMP

Recently, more and more attention has been paid to the preparation and application of polishing material due to the severe demand of chemical mechanical polishing for various optic lens and integrated circuit (IC) silicon wafer with high precision. In this paper, some methods were adopted to prepare ceria-based polishing powders for the purpose of enhancing their polishing performance and economic feasibility.Ceria nanoparticles were synthesized simply by pyrolysis method using large crystalline hydrate cerium propionate as precursor, which was prepared by normal crystallization method after reaction between cerium carbonate and propionate acid. The effects of pyrolysis temperature on the physical properties of ceria were investigated. It was found that the large crystals of precursor cracked to many nano-sized ceria particles on heating, and the medium particle sizes D₅₀ determined by laser scattering (LS) method decreased and then increased with minimum value around 460nm at calcination temperature of 1000℃. XRD, SEM and TEM observations showed that the average particle size of synthesized ceria powders ranged from 20 to 30nm. The removal rate of optical glasses K₉ and F₁ polished with ceria powders synthesized at different calcinations temperatures showed a maximum value at temperature of 1000℃. This is corresponding to the minimum value of particle size. Furthermore, the removal rates of copper slip were also evaluated, and varied with the polishing pressure, rotation speed, and the concentration of both ceria particles and oxidant.CeO₂ particles doped with B or Ti were prepared by calcining hydrate cerium propionate mixed with boric acid or titanium sulfate. It was found that the doping of B or Ti in CeO₂ resulted in particle glomeration and improved the polishing ability. Similar results were also observed for nano-sized Ce_1-xTi_xO₂ synthesized by co-precipitation method with cerium nitrate, titanium sulfate and ammonia as raw materials. These facts indicate that doping with some elements in ceria is a simple route to prepare ceria-based oxides with sphere morphology and strong polishing ability.Ultra fine CeO₂-ZrO₂ mixed oxides were prepared by milling solid cerium carbonate and zirconium oxy-chloride with ammonia and followed by filtering, drying and calcining procedures. The results show that the mixed oxide calcined at 1000℃is composed of cubic ceria doped with zirconium and tetragonal zirconia doped with cerium, and the phase composition varies with calcination temperature and the Ce/Zr molar ratio. The monoclinic zirconia was observed when decreasing calcination temperature and shortening milling time, demonstrating that milling and calcining could force the phase transformation from monoclinic zirconia to cerium stabilized tetragonal zirconia and zirconium doped cubic ceria solid solutions. The removal rate for the optical glass polishing varied with Ce/Zr molar ratio. A synergetic polishing effect was found when Ce/Zr molar ratio below 4, and the optimal Ce/Zr molar ratio is 1:1. At the same time, the polishing property of the synthesized ceria-zirconia mixed oxide is also affected by their particle physical characteristics.Ceria slurries were prepared by calcining cerium carbonate directly at 1000℃and following by milling in water containing dispersant sodium hexametahposphate (NaP), or PEG , or PAA or their mixtures of (NaP+PEG), (NaP+PAA). The suspension stability of ceria particles in slurries and the pH dependence were examined by means of Zeta potential determination. The removal rate of different slurries for silica wafer or optical glasses polishing were also been investigated. It was found that the removal rate was proportional to the suspension ability of slurry, which is dominated by the dispersants added. It is better when two different kinds of dispersants were adopted, such as the cases of NaP+PEG or NaP+PAA.